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Saturday, April 14, 2007

Black Hole Session

I mentioned some weeks ago that I volunteered for PI's public lecture series 'Black Hole Session'. Originally, I was scheduled for the first Saturday in April, but then somebody from human resources figured out this was Easter, and so I was postponed to this weekend. This turned out to be really fortunate, since I was sick last week and the only noticeable activity I achieved was my nose evolving from a white dwarf to a red giant.

Still, I was really running behind with preparing the talk. The problem was when I realized I actually had time to talk about whatever I wanted, I could not decide on what to talk about. Then I recalled what I found the most exciting when I was a student were not all the things that were known, but the open problems that were left for us to explore. So I thought, instead of talking about all the things we know I would - somewhat unusual for a scientific talk - focus on what we don't know, and where the frontiers of our knowledge currently are. Thus, the title of the talk

Frontiers of our Knowledge

Abstract: Theoretical and experimental physics work hand in hand to broaden our understanding about the universe that we live in and man's place in the world. In the 21st century, nature has given us quite some puzzles to solve, in the microscopic (particle physics) as well as in the macroscopic (cosmology) range. These open questions at the threshold of the unknown have lead theoretical physicists to formulate possible solutions whose experimental tests are awaited soon. I will talk about these current limits to our knowledge, and about the insights that new experiments like the Large Hadron Collider can provide us with. A central point will be the possibility of large extra dimensions and black hole production at the LHC.

If you want to classify my current state of mind, I'd say I'm an high energy physicist trying to become a cosmologist. I think that in the soon future more interests will shift from particle physics towards cosmology, which right now I find a tremendously exciting area. So in my talk I wanted to talk about both, cosmology and particle physics, especially also the areas where they overlap e.g. dark matter searches, and what that has to do with the 'big' questions like: Where do we come from? What are we made of? (Why am I spending my Saturday at work?)

Well, at least that was the idea. But I have never before given a talk about cosmology (I don't even know what an 'erg' is. Luckily, nobody asked.) I was really kind of nervous (in addition you should know that this lecture starts at 10am, and I didn't have any coffee because due to some problem with the key cards I couldn't get into my office.)

PI's public outreach program is organized by Damian Pope, who told me the format is rather casual, and the physics knowledge of the audience often pretty mixed. So I thought the best would be not to use too many equations, but to really explain every detail (be honest, usually you don't do that since everybody has seen this figure a million times). Bruno made a very nice introduction, and I looked at the large seminar room getting fuller with people. You're not going to believe it, but soon all seats were taken. In fact, after 15 minutes, Damian asked me to interrupt my talk so we could change into the big lecture hall. As I said to Bruno: I think I'm in the wrong movie.

As you can guess, my timing for the talk was a complete disaster. I had to skip the biggest part of the second half, and I had to promise I would put the slides online, so here they are:

People had a lot of questions. The best question was without doubt: Do you know what caused the big bang? It was a good question, because it didn't ask what caused it, but if I know it. It happens only rarely that I can clearly answer a question with yes or, in this case, with no.

If I find the time, I will write a summary of the talk sometime next week, if you will find it, then you will find it here.

Overall seen, giving the lecture was a great experience. In fact, I volunteered to do it again...

34 comments:

So what's wrong with this pictureWhy does it emerge in one direction and not in anyone of another 360x360 directions.

And what is it emerging into, nothingness? and we call those who believed the earth was flat, and you would fall off the edge if you travelled too far east or too far west, short-sighted?

I also believe in recycling, the universe is in a continuous (and perhaps even 'perpetual'?) state of recycling or recycle-ment

But best of all bee, when does gravity appear, when does energy become matter and particles reach mass of velocity, and cluster into gravity ... or was gravity all held in a singularity and dispersed thru the universe by one of these 'big bangs' occurring spontaneously because of some per chance quantum fluctuation.

How about this one for Lee,atoms & particles (matter) falling into a blackhole evaporates, the remainder given off as light.

But, as we know from shooting stars entering the earth's atmosphere, some small portion still reaches the ground.

Blackholes with a singularity, are there hidden in the universe.Blackholes with a singularity which come into contact with other galaxies can acrete matter & mass, until they become too voluminous, expand and disperse gravity, mass and matter (Recycled).

Blackholes with no singularity are mouths of wormholes, in this case it is not a massively dense singularity (with gravity) that is pulling matter into the blackhole, but rather a capillary-like effect if fluids.

My dust workshop in Heidelberg was an explosion of new ideas and connections because this was the first transformation into an entity that includes astrophysicists and cosmochemists and cosmologists.

Your Slide 20 of your presentation was also a view I saw last Wednesday at my workshop in a presentation by Valeri Dikarev. There exists in WMAP data still the issue of the local (solar system or galactic) dipole alignment. He's tried many kinds of local dust sources and transforming those sources into a WMAP data acquisition pattern, and, so far, nothing works to give a dipole alignment signature like what WMAP sees. It's a tough problem.

Your last slide gives Carl Sagan as the author of that quote, but I thought it was Isaac Asimov. Am I wrong?

Dusty Visions! I like that title :-) The quotation is by Asimov? Now that would be kind of embarrasing. I found it online, see e.g. here, but I admit I didn't check for a reference. Will try to find out - thanks for letting me know!Best,

well, there are limits to what you can visualize. Sometimes an equation says more than a picture, much as like the picture says more than a thousand words. It's really hard to draw a three dimensional volume expanding into all spatial directions. I still like best the comparison with an inflating ballon.

when does gravity appear, when does energy become matter and particles reach mass of velocity, and cluster into gravity ... or was gravity all held in a singularity and dispersed thru the universe by one of these 'big bangs' occurring spontaneously because of some per chance quantum fluctuation.

Though not everybody would agree with me I like to think gravity has been there 'all the time' (to circumvent speaking about a beginning of time). It boggles my mind to think about how a quantum can fluctuate if there is no time?

How about this one for Lee,atoms & particles (matter) falling into a blackhole evaporates, the remainder given off as light.[...] Blackholes with a singularity, are there hidden in the universe.

I believe the point of his black holes is that they have no singularity...

Hi Bee, the thing with a blackhole in space with no massively dense singularity to 'create' the gravity - is that it has to work with a capillary-like effect, in other words be a 'wormhole' to another part of space (the universe)conceptually he could have the whole universe going thru itself in a loop if he likes, but I'm concerned with 'commonal garden' blackholes

In hydrology, capillary action describes the attraction of water molecules to soil particles. Capillary action is responsible for moving groundwater from wet areas of the soil to dry areas. Differences in soil matric potential (Ψm) drive capillary action in soil.

This of course on earth, limited by the effects of earth's gravity. Translate it to space, where space is the soil, and water is the matter (particles)That is the visual which speaks a thousand words, and mathematicians should be able to equate (compute or formulate).

Yeah, the quote is great - its short, clear and motivational, I like it a lot. I admit though, I don't really care who said it one way or the other. It is somewhat annoying though that on the web all kinds of things get attributed to people without naming sources. I usually try to check that but I only added the quotation like 15 min before the talk. In particular, I repeatedly came across quotations that allegedly are by Einstein, but I am very sceptic about that - at the very least they sounded *oohm* very creatively translated.

Hi Quasar,

this is an interesting analogy indeed! But for the capillary effect, doesn't one need a boundary? What is the boundary in this case? The horizon?

As far as I understood, Hawking Evaporation works as some virtual particle pairs apear near the BH. Out of those pairs some single particles fall into the BH before meeting their twin to be anihilated. The rest escapes as radiation. But..

What if the black hole is too small? that is if the particles are too big to swallow or too hard to bite.

The microBH should do the same trick as the collider itself with all its might and power, cracking up the particle and eating bit by bit so to say.

yes, I know that picture for Hawking creation, but I don't particularly like it. I always found it somewhat confusing, because the infalling particle needs to have negative energy (how else could it decrease the mass of the black hole?). But yes, the question how small a black hole can possibly get is a good one. Let me first state that the black hole doesn't only emit massive particles, but all kinds of stuff, that is also radiation and gravitons.

Hawking's radiation is done using QFT in curved space. That is, particles are point particles. What is the extension of a point?There are some people who have argued the black hole will actually stop radiating and form a stable 'remnant' that can not emit anything (if you like, because it's too small so the creation can't take place - on can formulate that via the uncertainty principle). I myself have a paper on that, but it's not a work I'm particularly proud of (it's not technically wrong, but I missed a point, so its kind of inconsistent with its own idea).

The truth is that nobody really knows what happens in this very last stages of the Hawking evaporation. My view on the matter is the following: for creation of black holes from point particles one needs a threshold. The reason is again: point particles, when collided will ALWAYS form a black hole, even at arbitrarily small energies. So, one says, one needs a minimal energy scale to make the stuff collapse. But I would say, if the stuff can collapse and form a black hole, then the black hole should also be able to decay into that stuff (I know there is no reason to believe in time reversal symmetry in this case, but I do nevertheless). One way or the other, what one really needs to address the question is a quantum mechanical description of the black hole that we don't presently have. And we can learn that point particles are an annoyance, so how about we try one-dimensional objects...?

Visible matter suffered gravitation, collision, and presumably magnetic fields and plasma dynamics. The dark matter masses should have been gravitationally skewed by passing through each other and by adjacent visible matter distributions being reshaped, yes?

Very interesting slides, thanks for making them available. I like the can of Fermilab's primordial soup ("ylem", right?)!

Regarding the theoretical explanations for dark matter, you mentioned WIMPs, but I recall hearing about MACHOS (Massive Compact Halo Objects) some time ago as well. Are MACHOs pretty much ruled out now (who comes up with these silly acronyms anyway? But they certainly are easy to remember!)

Also, I recall reading something R. Penrose wrote sometime ago in "The Emperor's New Mind" regarding the scale of quantum gravity effects. He had speculated that these effects may be observable at the (incredibly) large scale of the order of ...micrometers! Don't know if he stil holds those views.

sorry for the delayed reply. The existence of MACHOS is not ruled out, but its pretty much ruled out that they constitute the dark matter. They could be a fraction of it though. You might find this site very helpful

http://www.daviddarling.info/encyclopedia/D/darkmat.html

Big Bang nucleosynthesis (BBN) predicts the right ratios for the elements in the universe today only if the original amount of baryonic matter was no more than 10% of the critical amount of matter needed to stop the cosmic expansion. Because scientists believe dark matter makes up far more than 10% of the critical value, they’re pretty confident that most dark matter isn’t made of baryons. [...]

Mention should also be made of MACHOs (Massive Compact Halo Object) – the name chosen, in 1991, deliberately to contrast with WIMP. According to the MACHO point of view, sizeable galaxies like our own are cocooned by dark matter haloes that have a hefty population of non-luminous objects such as brown dwarfs, other types of very dim stars, black holes, planets and so on. Although we wouldn’t be able to see MACHOs directly, we could, astronomers realized, hope to detect them in another way. Any concentration of matter can, under the right circumstances, act like a lens, bending and focusing the light rays from a source that lies behind them at a much greater distance. Since the early 1990s, a number of projects have been on the lookout for microlensing events that would give away the presence of MACHOs. Several extrasolar planets have been found by this technique. But, overall, not enough MACHOs have been found to account for more than a fifth of all dark matter, and the final figure may be much less. Also, because MACHOs are presumed to be made of ordinary (baryonic) matter, they’re contribution is likely capped by the BBN restriction mentioned earlier.

very good question. The presently best candidate for dark matter are so-called WIMPs that are weakly interacting massive particles. These do clump into dense structures, but since they are very weakly interacting, they don't form starlike things like usual matter does. They don't collide with each other and slow down, but basically move through each other. That is, they clump far less on smaller scales and are believed not to form (many) black holes.

A black hole is a black hole is a black hole. That is to say once it is formed, it doesn't matter what it was formed from. It is characterized only by its mass, electric charge, and angular momentum (that is the so-called no-hair-theorem). All black holes are believed to make Hawking radiation. For astrophysical objects though, the temperature is far too low to be detected.

yes, in principle dark matter can fall into a black hole. If it's a WIMP in this regards it isn't different from any other kind of matter. And if it does so, it gets 'converted' into Hawking radiation. Indeed, WIMPs should also be emitted as well in the Hawking radiation (though the fraction would be extremely tiny, as the black hole's temperature would typically be much much below the Massive particle's mass and so the probability for its emission is exponentially suppressed).

As I've mentioned above, WIMPs can 'in principle' form black holes. But unlike usual matter, the dark matter's interaction is extremely weak. It does not heat up, it does not slow down, it has very little friction and radiation loss. This is why it 'passes right through' another cloud of that stuff. It is therefore far less likely to clump and collapse. That doesn't mean it is impossible though, it just doesn't happen as often as for usual matter.

First, I am sorry about initially using completely unrelated thread. Maybe this one suits better.

My question was related to the way black holes are created when for example large enough star collapses.

I am assuming that the a sphere would do modeling the star, including considering the gravity field. My next assumption is that the gravity field inside a spherical object drops linearly towards the mass center where it reaches zero.

If the above consideration is true, then shouldn't the shape of the black hole be like a shell around where the gravity field is strongest, at least soon after the birth of the black hole?

A number of questions would follow, but I might have gotten something wrong by now. And I don't know if it makes any sense to think about 'topology' of 'singularity' - shouldn't those be inherently incompatible?)

Thanks, it is a good question. I think though you got something wrong there. If you like you can consider a mass shell collapsing. The gravitational field inside this shell doesn't only drop to zero towards the center, it is zero. It's the same as in Newtonian gravity. However, the shell is subject to the gravitational force and the point is that once the radius of the shell falls below the Schwarzschild radius* it is impossible to stabilize the matter, and it MUST collapse down towards a singularity. At least in classical GR. That's what all the singularity theorems are about. It is believed by many people that the actual formation of the singularity is avoided by some quantum gravitational effects, there are models that seem to confirm this, but at this stage nobody knows for sure.

Does that answer your question? There is no 'shape' of the black hole, matter can not exist in some 'shape' below the Schwarzschild horizon, it has to fall towards the singularity. All remaining shapes need to be radiated off in multipol moments (that is the `no-hair theorem', you have probably heard of it). You find a very nice explanation of this in MTW, if you have the book at hand. Best,

“There is no 'shape' of the black hole, matter can not exist in some 'shape' below the Schwarzschild horizon, it has to fall towards the singularity.”

Now it’s true that I know nothing about the quantum considerations beyond perhaps “Hawking’s Radiation”; coupled with the fact that I’m certainly not an expert in any of this. This means of course I too most likely have things confused. However, I was always under the impression that besides gravity, black holes could also exhibit two other physical qualities or attributes if you prefer; being charge and angular momentum. I’m aware that it has been shown that charge would quickly dissipate, yet I was under the impression angular momentum would remain. So then, to my point. I always thought that a rotating body collapsing to form a black hole would not have a singularity that was a simple point object, yet rather that as the result of the angular momentum it maintained (conserved) it would be bulged out like a disc. Also, I was under the impression that even when things passed below the event horizon that it had no necessarily immediate effect on the shape of body that enters, even less if the initial mass from which it was formed was great. This I know all changes however as the body approaches the singularity. You also have me intrigued that quantum theory seems to indicate that no singularity actually forms. I recall years ago Russian researchers insisting on much they same thing in referring to such entities as “frozen stars” as opposed to black holes. Of course depending on your point of view (from this side of the event horizon), or our side of the looking glass (as to time), the singularity never does form.

Yes, I was talking about a non-rotating shell that collapses. You are right, if the black holes has an angular momentum the space-time structure becomes more involved. In the standard picture the observer neither sees the black hole form, nor the singularity. I know that there are scenarios in which people argue spacetime doesn't even have a horizon, but for beginners I'd say one should first try to understand the conservative picture, before one starts playing with the less accepted alternatives. Best,

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